The difference and usage examples of define and defined functions in PHP

define

define is used to define a constant, which is also globally scoped. It can be accessed anywhere in the script regardless of scope. Once a constant is defined, it cannot be changed or undefined.

Constants and variables have many similarities, so they are easy to confuse; below, we list the differences between constants and variables:

1. A constant value cannot be changed after it is specified

2. When setting a constant, there is no need to add the "$" symbol in front

3. Constants can Accessed by all ranges of domains

4. The value of a constant can only be "string [string]" and "number [number]"

The syntax format of define is as follows:

define(name,value,case_insensitive)

Parameter	Description
name	Required parameter. Specify the name of the constant
#value	Required parameters. Specify the value of the constant
case_insensitive	Optional parameter. Specifies whether the name of the constant is case-insensitive. If set to True, letters are not case-sensitive; if set to False, letters are case-sensitive. The default value is: False

Specify a constant (case-sensitive), the sample code is as follows:

<?php
define("GREETING","How are you today?");
echo constant("GREETING");
?>

Output the following results:

How are you today?

Specify a constant (case-insensitive), the sample code is as follows:

<?php
define("GREETING","How are you today?",TRUE);
echo constant("GREETING");
?>

Output the following results:

How are you today?

defined

defined is used to detect whether a constant is defined. If the constant exists, it returns true, otherwise it returns false. The syntax format of

defined is as follows:

defined(name)

Parameter	Description
name	Required parameters. Specify the name of the constant object

The sample code is as follows:

<?php
define("GREETING","How are you today?");
echo defined("GREETING");
?>

Output the following results:1

The defined tag of ThinkPHP template engine is used to determine whether a constant has been defined.

The defined tag usage is as follows:

<present name="常量">要输出的内容</present>

Use the defined tag in template/Tpl/default/Index/display.html as follows:

<defined name="SITE_NAME">网站名称：{*SITE_NAME}</defined>

If the judgment is not defined, you can Use:

<notdefined name="SITE_NAME">{*SITE_NAME}不存在活未定义</notdefined>

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